The present invention relates generally to processing of semiconductor devices, and specifically to methods and apparatus for removal of foreign particles from semiconductor wafers and masks.
Removal of contaminants from semiconductor wafers is a matter of great concern in integrated circuit manufacturing. As the critical dimensions of circuit features become ever smaller, the presence of even a minute foreign particle on the wafer during processing can cause a fatal defect in the circuit. Similar concerns affect other elements used in the manufacturing process, such as lithographic masks.
Various methods are known in the art for stripping and cleaning foreign matter from the surfaces of wafers and masks, while avoiding damage to the surface itself.
For example, U.S. Pat. No. 4,980,536, whose disclosure is incorporated herein by reference, describes a method and apparatus for removal of particles from solid-state surfaces by laser bombardment. U.S. Pat. Nos. 5,099,557 and 5,024,968, whose disclosures are also incorporated herein by reference, describe a method and apparatus for removing surface contaminants from a substrate by high-energy irradiation. The substrate is irradiated by a laser with sufficient energy to release the particles while an inert gas flows across the wafer surface to carry away the released particles.
U.S. Pat. No. 4,987,286, whose disclosure is likewise incorporated herein by reference, describes a method and apparatus for removing minute particles (as small as submicron) from a surface to which they are adhered. An energy transfer medium, typically a fluid, is interposed between each particle to be removed and the surface. The medium is irradiated with laser energy and absorbs sufficient energy to cause explosive evaporation, thereby dislodging the particles.
One particularly bothersome type of contamination that is found on semiconductor wafers is residues of photoresist left over from a preceding photolithography step. U.S. Pat. No. 5,114,834, whose disclosure is incorporated herein by reference, describes a process and system for stripping this photoresist using a high-intensity pulsed laser. The laser beam is swept over the entire wafer surface so as to ablate the photoresist. The laser process can also be effected in a reactive atmosphere, using gases such as oxygen, ozone, oxygen compounds, nitrogen trifluoride (NF3), etc., to aid in the decomposition and removal of the photoresist.
European patent publication EP 0 943 390 A2, whose disclosure is incorporated herein by reference, describes a method of surface treatment using multi-laser combustion, providing an improvement on the method of the above-mentioned U.S. Pat. No. 5,114,834. The mechanism of photoresist removal is described as including a localized ejection of the photoresist layer, associated with a blast wave due to chemical bonds breaking in the photoresist and instant heating of the ambient gas. A fast combustion flash of the ablation products occurs due to the photochemical reaction of ultraviolet (UV) laser radiation and the process gases, which may also be due to the blast wave. The combination of laser radiation and fast combustion provides instantaneous lowering of the ablation threshold of hard parts of the photoresist.
Another method for photoresist removal is described in an article by Y. P. Lee, et al., entitled xe2x80x9cSteam Laser Cleaning of Plasma-Etch-Induced Polymers from Via Holes,xe2x80x9d in the Japanese Journal of Applied Physics 37 (1998), pp. 3524-3529, which is incorporated herein by reference. An alcohol film is applied to the insides of the via holes prior to irradiation by an excimer laser. Explosive evaporation of the alcohol caused by the laser pulse was found to improve the efficiency of removal of the photoresist from the holes.
PCT patent publication WO 96/06694, whose disclosure is incorporated herein by reference, describes a method for performing surface processing of flat panel display substrates, and particularly for removing photoresist after a photolithography step. A high-intensity pulsed beam of radiation is swept across the surface while a reactive gas is flowed across it. The radiation causes the photoresist material to react with the gas, resulting in gaseous products that are then drawn away from the surface. U.S. Pat. Nos. 5,669,979 and 5,814,156, whose disclosures are likewise incorporated herein by reference, describe further methods of cleaning a substrate surface photoreactively, in a manner that is purported to avoid damaging the surface. A laser beam of UV radiation is swept across the surface, while a flow of a reactant gas is provided in a reaction region so that the gas is excited by the UV laser beam.
U.S. Pat. No. 5,023,424, whose disclosure is incorporated herein by reference, describes a method and apparatus using laser-induced shock waves to dislodge particles from a wafer surface. A particle detector is used to locate the positions of particles on the wafer surface. A laser beam is then focused at a point above the wafer surface near the position of each of the particles, causing an electrical breakdown in the gas above the surface that produces gas-borne shock waves with peak pressure gradients sufficient to dislodge and remove the particles.
It is an object of the present invention to provide methods and apparatus for efficient removal of contaminants from solid-state surfaces, and particularly for removal of microscopic particles from semiconductor wafers and other elements used in semiconductor device production. The wafers may be bare, or they may have layers formed on their surface, whether patterned or unpatterned.
It is a further object of some aspects of the present invention to provide methods and apparatus for removal of contaminants from wafer surfaces in a manner that reduces the likelihood of damage to the surface.
In preferred embodiments of the present invention, a laser beam excites a process gas in proximity to a wafer surface, for the purpose of removing contaminants from the surface. Excitation of the gas generates reactive species, typically atomic and/or molecular radicals, which rapidly oxidize the contaminants, facilitating their removal. This chemical reaction cooperates with gas-dynamic effects, such as a blast wave, created by the intense laser irradiation. These combined effects substantially reduce the laser energy flux that must be applied to the wafer surface itself in order to achieve complete removal of the contaminants, thereby reducing the likelihood that the surface will be damaged in the process.
In some preferred embodiments of the present invention, the process gas comprises ozone, which is preferably diluted with oxygen gas. Irradiation of the ozone with ultraviolet laser radiation causes rapid dissociation of the ozone molecules into oxygen molecules and atomic oxygen radicals. This rapid dissociation causes an exothermic chain reaction, resulting in a strong local blast wave, which dislodges contaminant particles from the wafer surface. The oxygen radicals generated in the blast aid in the combustion and removal of the particles. Preferably, the ozone concentration is sufficiently high so that most of the laser radiation is absorbed in the gas and converted to blast energy. Thus, relatively little of the radiation reaches the wafer surface, and the probability of thermal or radiation damage to the surface is reduced. Furthermore, because the blast wave is driven by dissociation, rather than by electrical breakdown as in laser cleaning methods known in the art, there is substantially no risk of electrical damage to the wafer due to charging of the surface.
In some preferred embodiments of the present invention, the process gas reacts with heated water vapor in proximity to the wafer to create molecular radicals. Preferably, the process gas comprises a fluorine compound, such as NF3, which forms reactive species that include hydrogen fluoride (HF). Alternatively, the process gas may comprise a compound containing oxygen or another element that reacts with the water to form oxidizing radicals. Typically, the water vapor is introduced into a chamber containing the wafer before the laser excitation is initiated. The vapor condenses on the wafer to form a thin water film, as described, for example, in the above-mentioned U.S. Pat. No. 4,987,286.
Laser irradiation causes explosive evaporation of the water on the wafer, facilitating both the removal of the contaminants from the surface and etching of the contaminants by the HF and/or other reactive species. Preferably, ozone is mixed with the fluorine compound, so that a blast wave is generated as described above, together with high temperatures that increase the rate of formation and reaction of HF. The combined blast wave and chemical effects thus provide more effective contaminant removal than non-reactive methods of laser-driven steam cleaning that are known in the art, particularly when there is chemical adhesion of the contaminants to the wafer surface. The methods of the present invention thus achieve contaminant removal with effectiveness comparable to xe2x80x9cwetxe2x80x9d processes, but without the need to immerse the wafer in aggressive solvents as is required by wet processes known in the art.
Although preferred embodiments are described herein with reference to semiconductor wafer processing, the principles of the present invention may similarly be applied to remove contaminants from surfaces of other types, such as flat panel displays and photolithography masks, as well as from electrically-conductive surfaces.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for removing contaminants from the surface of a substrate, such as a semiconductor wafer, including:
introducing an explosive medium into a vicinity of the substrate; and
directing a beam of electromagnetic energy toward the substrate, such that absorption of the electromagnetic energy causes the explosive medium both to generate a blast wave and to form reactive species, the blast wave and the reactive species cooperating to remove the contaminants from the surface substantially without damage to the surface itself.
Preferably, the beam of energy has a wavelength and energy flux selected so as to cause explosive decomposition of the explosive medium, substantially without dependence on photoionization due to the beam. Most preferably, the explosive medium includes ozone, wherein introducing the explosive medium includes introducing sufficient ozone to absorb most of the electromagnetic energy directed at the substrate. Further preferably, introducing the explosive medium includes introducing the medium in sufficient quantity so that the explosive decomposition is caused by absorption of the electromagnetic energy in the medium itself.
Alternatively, the explosive medium includes water, which evaporates explosively to generate the blast wave upon absorption of the electromagnetic energy, and the method includes adding a process gas in the vicinity of the substrate, which decomposes and reacts with the water to form the reactive species.
Preferably, directing the beam of electromagnetic energy includes directing a pulsed laser beam at the substrate. Most preferably, directing the pulsed laser beam at the substrate includes irradiating the substrate at an energy density below an ablation threshold of the contaminants. Optionally, directing the pulse laser beam at the substrate includes inducing a photochemical interaction due to absorption of the electromagnetic energy at a surface of the substrate, so as to cooperate with the blast wave and the reactive species in removing the contaminants.
In a preferred embodiment, introducing the explosive medium includes introducing the explosive medium into a container having an outlet in proximity to the substrate, and directing the beam of electromagnetic energy includes directing the beam into the container, so as to generate the blast wave and the reactive species at the outlet of the container.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for removing contaminants from the surface of a substrate, including:
coating the substrate with a layer of a fluid;
introducing a process gas into a vicinity of the substrate; and
directing a beam of electromagnetic energy at the substrate, such that absorption of the electromagnetic energy causes the fluid to evaporate explosively so as to dislodge the contaminants from the surface, and to react with the gas so as to generate reactive species, which react with the contaminants.
Preferably, the fluid includes water, and the process gas includes a fluorine compound, wherein the fluorine compound reacts with the water to form hydrogen fluoride. In a preferred embodiment, the fluorine compound includes nitrogen trifluoride.
Additionally or alternatively, the process gas includes an oxygen compound, preferably ozone.
Preferably, coating the substrate with the layer of fluid includes introducing water vapor into the vicinity of the substrate, so that the vapor condenses onto the substrate.
There is additionally provided, in accordance with a preferred embodiment of the present invention, apparatus for removing contaminants from the surface of a substrate, including:
a chamber, adapted to receive the substrate and having an inlet for introduction of an explosive medium therethrough into a vicinity of the substrate and an outlet for removal of the contaminants; and
a source of electromagnetic energy, configured to direct a beam of the electromagnetic energy into the chamber, such that absorption of the electromagnetic energy in the chamber causes the explosive medium both to generate a blast wave and to form reactive species, the blast wave and the reactive species cooperating to remove the contaminants from the surface substantially without damage to the surface itself.
In a preferred embodiment, the chamber includes a container, coupled to the inlet so as to receive the explosive medium, the container having an outlet in proximity to the substrate and including a window that is substantially transparent to the electromagnetic energy,
wherein the source of electromagnetic energy is configured to direct the beam through the window of the container, so as to generate the blast wave and the reactive species at the outlet of the container.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for removing contaminants from the surface of a substrate, including:
a chamber, adapted to receive the substrate and having an inlet for introduction therethrough of a fluid to coat the substrate and for introduction of a process gas into a vicinity of the substrate, and an outlet for removal of the contaminants; and
a source of electromagnetic energy, configured to direct a beam of the electromagnetic energy into the chamber, such that absorption of the electromagnetic energy in the chamber causes the fluid to evaporate explosively so as to dislodge the contaminants from the surface, and to react with the gas so as to generate reactive species, which react with the contaminants.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: